How long – I believe you can leave the money in the current plan unless your company has some rules limiting you.

No penalty or taxes to keep it there.

However, if you do take it out, and I would, do a direct rollover into an IRA. Use one of the major firms (I have mine with Vanguard and Fidelity) and they will take care of it for you. You do not want the funds sent to you. The payment should go directly to them. The primary reason you should move it to an IRA is that you will have many more choices than what is offered in your 401K.

Last point - don’t be too quick to move it if your employer pays into it based on some sort of end of year profit sharing type plan. At one time my employer would contribute an extra amount if the company had a good year.